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Age, extent and carbon storage of the central Congo Basin peatland complex

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Abstract

Peatlands are carbon-rich ecosystems that cover just three per cent of Earth’s land surface1, but store one-third of soil carbon2. Peat soils are formed by the build-up of partially decomposed organic matter under waterlogged anoxic conditions. Most peat is found in cool climatic regions where unimpeded decomposition is slower, but deposits are also found under some tropical swamp forests2,3. Here we present field measurements from one of the world’s most extensive regions of swamp forest, the Cuvette Centrale depression in the central Congo Basin4. We find extensive peat deposits beneath the swamp forest vegetation (peat defined as material with an organic matter content of at least 65 per cent to a depth of at least 0.3 metres). Radiocarbon dates indicate that peat began accumulating from about 10,600 years ago, coincident with the onset of more humid conditions in central Africa at the beginning of the Holocene5. The peatlands occupy large interfluvial basins, and seem to be largely rain-fed and ombrotrophic-like (of low nutrient status) systems. Although the peat layer is relatively shallow (with a maximum depth of 5.9 metres and a median depth of 2.0 metres), by combining in situ and remotely sensed data, we estimate the area of peat to be approximately 145,500 square kilometres (95 per cent confidence interval of 131,900–156,400 square kilometres), making the Cuvette Centrale the most extensive peatland complex in the tropics. This area is more than five times the maximum possible area reported for the Congo Basin in a recent synthesis of pantropical peat extent2. We estimate that the peatlands store approximately 30.6 petagrams (30.6 × 1015 grams) of carbon belowground (95 per cent confidence interval of 6.3–46.8 petagrams of carbon)—a quantity that is similar to the above-ground carbon stocks of the tropical forests of the entire Congo Basin6. Our result for the Cuvette Centrale increases the best estimate of global tropical peatland carbon stocks by 36 per cent, to 104.7 petagrams of carbon (minimum estimate of 69.6 petagrams of carbon; maximum estimate of 129.8 petagrams of carbon2). This stored carbon is vulnerable to land-use change and any future reduction in precipitation7,8.

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Figure 1: Location of the Cuvette Centrale wetlands, study sites and our peatland probability map.
Figure 2: Tree height, peatland surface and peat depth along 24 km of transects extending from the peatland edge to the interfluvial centre.

Change history

  • 01 February 2017

    The Acknowledgements section was updated.

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Acknowledgements

We thank the Wildlife Conservation Society Congo Programme for logistical support and the villages that hosted our fieldwork: Bokatola, Bolembe, Bondoki, Bondzale, Ekolongouma, Ekondzo, Itanga, Mbala and Moungouma. We thank F. Twagirashyaka, T. F. Moussavou, P. Telfer, A. Pokempner, J. J. Loumeto and A. Rahïm (logistics); R. Mbongo, P. Abia (deceased), T. Angoni, C. Bitene, J. B. Bobetolo, C. Bonguento, J. Dibeka, B. Elongo, C. Fatty, M. Ismael, M. Iwango, G. Makweka, L. Mandomba, C. Miyeba, A. Mobembe, E. B. Moniobo, F. Mosibikondo, F. Mouapeta, G. Ngongo, G. Nsengue, L. Nzambi and J. Saboa (field assistance); M. Gilpin, D. Ashley and R. Gasior (laboratory assistance); D. Quincy (remote sensing and GIS support); D. Harris, J. M. Moutsambote (plant identification); P. Gulliver (radiocarbon analyses); F. Draper (access to Peruvian data); and T. Kelly and D. Young (discussions). The work was funded by Natural Environment Research Council (CASE award to S.L.L. and G.C.D.; fellowship to E.M.; NERC Radiocarbon Facility NRCF010001 (alloc. no. 1688.0313 and 1797.0414) to I.T.L., S.L.L. and G.C.D.); Wildlife Conservation Society-Congo (to G.C.D.), the Royal Society (to S.L.L.), Philip Leverhulme Prize (to S.L.L.), and the European Union (FP7, GEOCARBON to S.L.L.; ERC T-FORCES to S.L.L.). JAXA, METI, USGS, NASA and OSFAC are acknowledged for collecting and/or processing remote sensing data.

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Authors

Contributions

S.L.L. conceived the study. G.C.D., S.L.L., I.T.L., S.A.I and S.E.P. developed the study. G.C.D. collected most of the data, assisted by B.E.Y., S.L.L. and I.T.L. Laboratory analyses were performed by G.C.D. G.C.D. and E.T.A.M. analysed the remotely sensed data. G.C.D., S.L.L., I.T.L., E.T.A.M. and S.E.P. interpreted the data. G.C.D. and S.L.L. wrote the paper, with input from all co-authors.

Corresponding author

Correspondence to Greta C. Dargie.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks J. Chambers, L. Fatoyinbo and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Peatland water table time-series data.

a, b, Time series of water-table levels for the Ekolongouma (a) and Itanga (b) transects for the time period March 2013 to May 2014 (black, blue and red lines indicate different sample locations along the transects). c, d, Time series of water-table levels for the wet-season month of October 2013 for the Ekolongouma (c) and Itanga (d) transects, when river-caused flood events are more likely (left-hand axis; black, blue and red lines), and daily TRMM rainfall estimates (right-hand axis; purple lines). No obvious flood waves are seen. e, Relationship between the summed monthly cumulative increase in water table (CIWT) from 10 pressure transducers (Itanga, Ekolongouma, Bonzale and Bondoki transects), for months in which CIWT > 0, and summed monthly rainfall estimates for the same months from TRMM (best-fitting line: y = 0.959x − 133, R2 = 0.90, P < 0.001). Months during which the water table was not always above the peatland surface (CIWT ≤ 0) were excluded from the analysis, owing to large changes in the water table that obscure the relationship between water table and water input. Data from 10 pressure transducers are included, because two transducers had no months during which the water table was consistently above the peat surface.

Extended Data Figure 2 Spatial distribution of the ground-truth points across the Cuvette Centrale.

Main panel, ALOS PALSAR imagery of the Cuvette Centrale area and the spatial distribution of the ground-truth points (crosses for GPS, circles for Google Earth derived points) that were used as test and training data in the 1,000 runs of the maximum likelihood classifications used to estimate regional peat extent. The black boxes correspond to the other panels: a, the main study region; b, c, two regions within DRC where GPS ground-truth points were also obtained.

Extended Data Figure 3 Relationship between estimates of peat depth using the field-pole method and those using peat cores followed by laboratory analysis, and the relationship between corrected peat depth and total peat carbon stocks.

a, Relationship between peat depth (in m) estimated using a metal pole (rapid protocol) and estimated using coring and laboratory analysis (full protocol); LOI, loss-on-ignition; best-fitting line: y = 0.888x − 34.8, R2 = 0.97, P < 0.001, where y is cored peat depth and x is pole peat depth. The organic matter content of the core must be ≥65% to be classified as peat. Soft carbon-rich material that is <65% organic matter is captured using the rapid protocol, which lies beneath peat using our definition, but above the more typical mineral soil. b, Relationship between core depth (in m) and total carbon stocks (in Mg C ha−1) for cores from the Cuvette Centrale (best-fitting line: carbon stocks = 1,374 + 2,425log10(total core depth), R2 = 0.89, P < 0.0001).

Extended Data Figure 4 Distribution of peatland carbon stock estimates.

Estimated carbon stocks from 100,000 resamples of peatland area, peat depth and per-unit-area carbon storage. Median, 30.6 Pg C; mean, 29.8 Pg C; 95% CI, 6.3–46.8 Pg C.

Extended Data Table 1 Description of remote-sensing products used to identify field sites in the Cuvette Centrale
Extended Data Table 2 Radiocarbon dates from nine peat cores
Extended Data Table 3 Average peat accumulation rate and long-term rate of carbon accumulation (LORCA) for nine radiocarbon-dated peat cores
Extended Data Table 4 Vegetation classes encountered in the field, and their associations (or not) with peat
Extended Data Table 5 Remote-sensing products used in the maximum likelihood classification to map peatland extent within the Cuvette Centrale
Extended Data Table 6 Land-cover classes, ground-truth sample sizes, estimated extent of each class from 1,000 maximum likelihood model runs, and producer’s, user’s and overall accuracy of the classifications

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Dargie, G., Lewis, S., Lawson, I. et al. Age, extent and carbon storage of the central Congo Basin peatland complex. Nature 542, 86–90 (2017). https://doi.org/10.1038/nature21048

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